In the field of iron and steel-making, melting containers, also called melting furnaces, for the production of liquid metal, are well known.
Melting furnaces are typically fed with solid materials that contain a high concentration of the metal to be produced. The final composition of the liquid metal is adjusted by adding other metal or non-metallic compounds and very often with materials that have a high carbon content.
Melting and refining furnaces can generally be divided into two types:
electric furnaces that use electric energy as a source of additional energy to the chemical energy generated by the melting processes;
heat furnaces that do not use electric energy and use only heat sources; if additional energy is required in addition to the chemical energy produced by the refining reactions, burners are used for example.
The types of electric furnaces most often used can comprise electric arc furnaces, induction furnaces, resistance furnaces. A variant of electric arc furnaces, with regard to the production of iron alloys, is the submerged arc furnace.
Heat furnaces can comprise oxygen converters, Martin-Siemens furnaces and cupola furnaces.
After the primary melting step of the metal, a refining step is provided. Usually different alloy materials are added to the molten material, to obtain the required chemical composition.
In all these processes, during the melting and refining steps, all or only some of the materials to be melted can be loaded. The melting and refining steps are also called “power on charging”, and correspond to the time during which heat/electric energy is supplied to the melting furnace.
During this step the materials can be loaded into the melting furnace by a conveyor that adjusts the delivery rate thereof.
During the power on charging, the limitation of the delivery rate depends on the requirements of heat energy specific for melting: the higher the specific melting energy required, the lower will be the delivery rate and the productivity of the melting furnace.
In order to reduce the duration of the melting process, it is also known to heat the material before it is loaded into the melting furnace.
One well-known technology for heating the metal material exploits the principle of magnetic induction.
The magnetic induction heating technique is normally used in the fields of melting, heat treatments, molding and welding.
One example application of the magnetic induction heating technique, regarding the melting process of metal materials, is the induction furnace.
An induction furnace can consist of a melting container, usually made of ceramic material, with a cylindrical shape and to which induction heating devices are associated. The induction heating devices normally comprise at least one coil disposed around the melting container, which is powered by alternate current at a suitable frequency. The coil can consist of a tube wound in spirals in which a cooling fluid, usually water, is made to circulate, to preserve the properties of mechanical resistance thereof.
The alternate electric current circulating in the coil generates an alternate induced magnetic field in the melting container and generates induced currents in any conductive metal material that is struck by the magnetic field induced. The currents induced in the conductive metal material in their turn generate heat energy due to the Joule effect.
With regard to the technique of heating a metal mass by induction, the metal product can be heated by means of longitudinal flux induction or transverse flux induction.
In the case of longitudinal flux induction, the coils and magnetic yokes, which make up part of the heating devices, are disposed in such a way as to concentrate the magnetic field induced along the longitudinal development of the material to be heated. In this case the heating of the metal material occurs along the axis of the coil. A similar example of heating the metal material, by the action of the longitudinal magnetic field, is the one which occurs in an induction melting furnace.
In the case of transverse flux induction, the components of the heating device are disposed on opposite sides of the metal material to concentrate the oscillating magnetic field through the material to be heated. In this case, the main heating action occurs on the surface of the metal material.
With regard to the heating of the metal material before it is loaded into a melting furnace, a process and a plant are known, for example from U.S. Pat. No. 4,403,327, in which the scrap or other metal material is first heated by induction in a first container and is then loaded into a second container to be melted, again exploiting the induction principle.
The metal material exiting from the first container is still in a solid state and is transferred to the second container for example by loading baskets.
The loading baskets provide a direct, uncontrolled and immediate feed of the metal material directly into the second container.
This is particularly disadvantageous because it is not possible to suitably and continuously control the ways in which the metal material, inserted into the furnace during the whole melting process, is introduced.
In fact, a continuous control of the quantity of material inserted would prevent the mass of already molten metal from being subjected to drastic reductions in temperature.
Document U.S. Pat. No. 7,905,940 instead describes a continuous refining method for metal materials that provides to use an inductor coil to heat and melt the metal material contained in a melting container in order to reduce the metal oxides and vitreous components. Gas can be injected into the metal mass subjected to the refining process in order to produce, in the metal mass, the desired reduction effect. In this case, the metal material is both loaded, heated and melted in the melting container. The process and plant described are particularly complex and difficult to manage because, in the same container, zones are generated in which the material is still solid and zones in which the material is already in a molten state. This leads to a different distribution of the temperature in the container and therefore different critical states can occur in every zone of the container, due for example to the different types of materials used to make the container, the different types of cooling devices, different wear phenomena, and necessary maintenance operations not simultaneous in the various zones, or suchlike.
Document DE 30 26 720 A1 describes an induction heating device for metal material used to melt the material and equipped with a selective closing device that can be opened to cast the molten metal into an ingot mold below.
Document WO 2008/087244 describes a pre-heating device for metal material that uses a plurality of silos fed by a mixture of hot gases. Each silo is connected to the melting furnace by respective feed pipes. The gas cooling system is not very controllable and can lead to the melting of the material, in particular for small pieces.
Document U.S. Pat. No. 4,403,327 A describes an electric power system for an induction heating furnace. The induction furnace has an opening bottom, directly connected to a scrap container sliding on a slider, by means of which the electric melting furnace is fed.
One purpose of the present invention is to obtain a plant to melt metal material that allows to reduce the times of the production cycle.
Another purpose of the present invention is to obtain a plant for melting metal material that allows to reduce the complexity of the plant and its manufacture.
Another purpose of the present invention is to perfect a method to melt metal materials that allows to reduce the times of the melting cycle.
Another purpose of the present invention is to obtain a heating apparatus for metal products, installable in a melting plant, that guarantees to feed the metal material to the melting container in predetermined modes.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.